Electron gun device of field emission type

ABSTRACT

An electron gun device of the field emission type has an enclosure, at least a part of which is of non-magnetic material, an accelerating electrode having a first and second anode each comprising a magnetic material and disposed in said enclosure, and a focusing lens disposed outside the non-magnetic part of said enclosure, wherein gas is evacuated from said enclosure whereby a high intensity electron beam is obtained.

United States Patent [191 Katagiri et al.

[45.1 May 14, 1974 ELECTRON GUN DEVICE OF FIELD EMISSION TYPE lnventors: Shinjiro Katagiri, Hachioji; Akira Tonomura, Tokyo, both of Japan Assignee: Hitachi, Ltd., Tokyo, Japan Filed: Dec. 23, 1971 Appl. No.: 211,201

Foreign Application Priority Data Dec. 24, 1970 Japan 45-116718 US. Cl 313/82, 250/311, 313/84,

313/336 Int. Cl. H0lj 31/00 Field of Search 250/31 1; 313/336, 84, 82

References Cited UNITED STATES PATENTS 4/1973 Smith et a1 313/336 X 10/1930 Ackermann 313/84 X 4/1949 Liehmnnn 313/84X 2,619,607 11/1952 Steers v. 313/84 3.678.333 7/1972 Coales et a1. 313/82 R X FOREIGN PATENTS OR APPL1CAT1ONS 1,012,040 12/1965 Great Britain 2511/3 11 Primary Examiner-Archie R Borchclt Assistant Examiner-Saxfie1d Chatmon, Jr. Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT An electron gun device of the field emission type has an enclosure, at least a part of which is of nonmagnetic material, an accelerating electrode having a first and second anode each comprising a magnetic material and disposed in said enclosure, and a focusing lens disposed outside the non-magnetic part of said enclosure, wherein gas is evacuated from said enclosure whereby a high intensity electron beam is obtained.

5 Claims, 2 Drawing Figures FIG. I PRIOR ART FIG. 2

INVENTORS SHINJIRO KATAGIRI AKIRA TONO RA BY c QMWQQQL- H4122 I ATTORNEYS ELECTRON GUN DEVICE OF FIELD EMISSION TYPE BACKGROUND OF THE INVENTION This invention relates to an electron gun device of 5 the field emission type, which is particularly suited for use as a source of electrons for an electron beam in an electron microscope and the like apparatus. I

As is well known, the conventional field emission type electron gun device used for electron microscopes comprises a needle-shaped cathode tip, accelerating electrodes, and a focusing lens for focusing electrons emitted from said tip into a fine beam.

This type of electron gun device requires an ultrahigh vacuum such as, for example, pressures below Torr, to maintain a stable field. If the vacuum is 10' Torr, the electron beam from said tip varies by l to 4 percent in terms of the current generated. At such vacuum, the field emission intensity decreases to make the electron gun device substantially impractical.

Another problem with this type of electron gun device lies in the fact that a large quantity of gas is to be evacuated by way of'the surface of said focusing lens which makes it difficult to maintain the vacuum higher than 10 Torr. To obviate the influence ascribable in the prior art manner of evacuating the gas, it has hitherto been the practice to take quite a long time to evacuate the gas.

SUMMARY OF THE INVENTION An object of the invention is to provide an electron gun device operable to produce a high intensity electron beam.

Another object of the invention is to provide an electron gun device readily capable of providing a very high vacuum.

A further object of the invention is to provide an electron gun device which can be constructed into a small size.

A still further object of the invention is to provide an electron gun device suited for use as a source of electrons for an electron beam in the electron microscope and the like.

Briefly, the above objects are accomplished in accordance with aspects of the present invention by the provision of an electron gun device in which the accelerating electrode is made of a magnetic material, the walls forming a space in which said accelerating electrode is disposed are made of a non-magnetic material, and said focusing lens is disposed outside said non-magnetic member so that the magnetic flux from the focusing lens is introduced into said accelerating electrode.

The construction, operation and effects of the present invention will be described in detail hereinafter in comparison with the prior art with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING ment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is shown a conventional electron gun device of the field emission type in which the electron beam emitted from a tip 2 by field emission in a vacuum-tight enclosure 1 is accelerated and converged by an accelerating electrode consisting of a first anode 3A and a second anode 3B, and then is focused by an annular focusing lens consisting of a cylindrical coil 4, a magnetic path 5 and a gap 6. In FIG. 1, an isolator 7 of the insulating material is interposed between the first and second anodes 3A and 3B.

In this electron gun device, because the focusing lens is located inside the enclosure 1 and the magnetic path 5 is of a ferrous metal or the like, the quantity of the gas evacuated through the surface of the magnetic path 5 is inevitably quite large. Because the gas is evacuated in such a manner, there are difficulties in keeping the space 1 at pressures below IO Torr despite the fact that the pressure must be below 10 Torr in order to maintain a stable field emission from the tip 2.

Assume that pressures of IO Torr could be obtained in the space 1. At such pressures, however, the electron beam obtained from the tip 2 by field emission varies in the range from 1 to 4 percent and, as a result, the intensity of the electron beam is reduced. To solve this problem, the quantity of the evacuating gas must be minimized.

With this requirement in view, the electron gun device in accordance with the present invention is so arranged that, as illustrated in FIG. 2, part of the enclosure 1 is constituted of a non-magnetic material 8, the first and second anodes 3A and 3B are made of a magnetic material, and a focusing lens is disposed in close contact with the outer side of said non-magnetic part 8 of the enclosure 1. In this construction, a magnetic path 5 is disposed about the coil 4 for the focusing lens so that said first and second anodes may be operable as pole pieces.

Thus, according to the invention, it is sufficient for the enclosure 1 to contain only the first and second electrodes 3A and 3B and insulator 7. This serves to make the quantity of evacuating gas smaller than in the prior art arrangement having the focusing lens in the enclosure 1. The invention also makes it possible to construct the electron beam accelerating parts to be integral with the focusing part, and to reduce the size of the device-as awhole. This is why the inner surface area of the enclosure 1' can be reduced and pressures of about l0 Torr can be maintained inside the spac thereof.

In the electron microscope or the like device asso-.

ciated with this invention, an intermediate lensand an objective lens and the like (not shown) are disposed after the stage formed by said second anode 3B (FIG. 2). The space (not shown diagrammatically) in which said lenses are located communicates with the space in the closure 1 accommodating the intermediate lens and other electrodes, through an aperture 9 provided in the second anode 3B. In this structure, the space accommodating the intermediate lens, etc., and the space inside the enclosure 1 can be considered to be independent of each other in view of the small size of aperture 9. Thus, a pressure difference can be maintained between said two spaces even if the space accommodating the intermediate lens, etc., is of such material as will allow a large amount of gas to escape. Hence, the space inside the enclosure 1 where the accelerating electrode is located is not affected by gas evacuation. In practicing the invention, for example, it is easily possible to obtain .pressures of 3X10 Torr inside the enclosure 3 1 and X10 Torr in the space accommodating the intermediate lens, etc., on the condition that the diameter of the aperture is lmm. In other words, a pressure difference of the order of tens of Torr between the two spaces can be easily obtained.

According to this invention, the magnetic flux produced in the coil 4 and magnetic path 5 of the focusing lens disposed outside the non-magnetic body 8 of the enclosure 1 is introduced through the non-magnetic body 8 into the first and second anodes 3A and 3B of magnetic material. Generally, the static electric field and the static magnetic field produce field distributions which are formed simultaneously and independently of each other. Hence, said anodes 3A and 3B become operable as an electric field lens and a magnetic field lens as well. Compared with the conventional device comprising a lens capable of forming only an electric field, the device having a lens capable of forming a magnetic field in addition to an electric field makes it possibleto reduce the amount of aberation in the electron beam by controlling the magnetic field. Thus, with the device of this invention, a high intensity electron beam can be easily realized.

As is apparent from the foregoing description, according to the present invention, ultra-high vacuum conditions can be obtained free of the influence caused by the evacuating gas, and'therefore the intensity of the electron beam can be markedly increased. The invention can be applied to all the electron gun devices of the field emission type which at least comprises a needle-shaped cathode tip, accelerating electrodes, and a focusing lens for thereby focusing the electrons into a beam.

What is claimed is: 1

1. In an electron gun device of the field emission type, a vacuum-tight enclosure made at least in part of non-magnetic material, means in said enclosure for generating an electron beam along a beam path by field emission, accelerating electrode means including a first anode and a second anode each made of magnetic material and being disposed in spaced relationship in said enclosure adjacent the non-magnetic portion thereof along said beam path for accelerating said electron beam, and focusing lens means disposed outside said enclosure adjacent the non-magnetic portion of said enclosure for focusing said electron beam, said nonmagnetic portion of said enclosure being disposed between said accelerating electrode means and said focusing lens means.

2. An electron gun device according to claim 1 wherein said first and second anodes are concentrically disposed within said focusing lens.

3. An electron gun device of the field emission type, a vacuum-tight enclosure made at least in part of nonmagnetic material, means in said enclosure for generuting an electron beam along a beam path by field emission, accelerating electrode means including a first anode and a second anode each made of magnetic material and being disposed in spaced relationship in said enclosure adjacent the non-magnetic portion thereof along said beam path for accelerating said electron beam, and focusing lens means disposed outside said enclosure adjacent the non-magnetic portion of said enclosure for focusing said electron beam, said focusing lens being formed by an annular coil and a magnetic cover surrounding all of said coil except an inner annular portion surrounding a central space which forms a lens gap the non-magnetic portion of said enclosure being disposed in said lens gapof said focusing lens.

4. An electron device according to claim 3 wherein said first and said second anodes are disposed to form extensions of said magnetic cover within said enclosure and thereby act as pole pieces for said focusing lens.

5. An electron gun device of the field emission type comprising: a vacuum-tight enclosure; means including a cathode tip for emitting an electron beam along a beam path by field emission in said vacuum tight enclosure, accelerating electrode means including a first and a second anode disposed in said enclosure along said beam path for accelerating the electron beam'emitted from said cathode tip, and focusing lens means including a cylindrical coil and a magnetic cover surrounding said coil and having a gap disposed therein for focusing the electron beam passing through said accelerating electrode means;

wherein at least a part of said enclosure is formed of a non-magnetic material, said first and second anodes being formed of a magnetic material and being disposed inside said enclosure adjacent the part of the enclosure made of non-magnetic material, and said focusing lens being disposed outside said enclosure adjacent the part of the enclosure made of non-magnetic material so that the nonmagnetic portion of said enclosure is disposed in said lens gap, whereby said first'and second anodes additionally operate as pole pieces for said focusing lens means. 

1. In an electron gun device of the field emission type, a vacuum-tight enclosure made at least in part of non-magnetic material, means in said enclosure for generating an electron beam along a beam path by field emission, accelerating electrode means including a first anode and a second anode each made of magnetic material and being disposed in spaced relationship in said enclosure adjacent the non-magnetic portion thereof along said beam path for accelerating said electron beam, and focusing lens means disposed outside said enclosure adjacent the non-magnetic portion of said enclosure for focusing said electron beam, said non-magnetic portion of said enclosure being disposed between said accelerating electrode means and said focusing lens means.
 2. An electron gun device according to claim 1 wherein said first and second anodes are concentrically disposed within said focusing lens.
 3. An electron gun device of the field emission type, a vacuum-tight enclosure made at least in part of non-magnetic material, means in said enclosure for generating an electron beam along a beam path by field emission, accelerating electrode means including a first anode and a second anode each made of magnetic material and being disposed in spaced relationship in said enclosure adjacent the non-magnetic portion thereof along said beam path for accelerating said electron beam, and focusing lens means disposed outside said enclosure adjacent the non-magnetic portion of said enclosure for focusing said electron beam, said focusing lens being formed by an annular coil and a magnetic cover surrounding all of said coil except an inner annular portion surrounding a central space which forms a lens gap the non-magnetic portion of said enclosure being disposed in said lens gap of said focusing lens.
 4. An electron device according to claim 3 wherein said first and said second anodes are disposed to form extensions of said magnetic cover within said enclosure and thereby act as pole pieces for said focusing lens.
 5. An electron gun device of the field emission type comprising: a vacuum-tight enclosure, means including a cathode tip for emitting an electron beam along a beam path by field emission in said vacuum tight enclosure, accelerating electrode means including a first and a second anode disposed in said enclosure along said beam path for accelerating the electron beam emitted from said cathode tip, and focusing lens means including a cylindrical coil and a magnetic cover surrounding said coil and having a gap disposed therein for focusing the electron beam passing through said accelErating electrode means; wherein at least a part of said enclosure is formed of a non-magnetic material, said first and second anodes being formed of a magnetic material and being disposed inside said enclosure adjacent the part of the enclosure made of non-magnetic material, and said focusing lens being disposed outside said enclosure adjacent the part of the enclosure made of non-magnetic material so that the non-magnetic portion of said enclosure is disposed in said lens gap, whereby said first and second anodes additionally operate as pole pieces for said focusing lens means. 